Volumetric breast ultrasound scanning usually involves a rectilinear movement of a linear-array ultrasound transducer relative to the breast tissue of a patient, with successive scanning lines parallel to one another, and processing of resultant ultrasound echoes to form a data volume representing local (e.g., voxel) values of at least one acoustic property of the scanned breast. Volumetric ultrasound scanning of the breast has been proposed as a complementary modality for breast cancer screening. One example is discussed in U.S. Pat. No. 7,828,733 and involves using a full-field breast ultrasound (hereinafter “FFBU”) scanning apparatus that chestwardly compresses a breast, and a rectilinear transducer translation mechanism that maintains an ultrasound transducer in contact with the breast, as discussed, for example, in WO 2007/014292, which employs scanning through a fabric material porous to an ultrasound coupling agent and has the advantage of reducing image artifacts such as those believed to be due to air bubbles.
One of the most important factors in breast ultrasound is image quality, which is generally defined by parameters such as image spatial resolution, signal dynamic range, and relative tissue image contrast. Image quality is very dependent on the frequency of the ultrasound. Major text books on breast ultrasound, such as “Breast Ultrasound” by A. Thomas Stavros (Publisher: Lipponcott Williams & Wilkins 2004) (hereinafter ‘Stavros 2004’) and “The Practice of Breast Ultrasound: Techniques, Findings, Differential Diagnosis” by Helmut Madjar and Ellen Mendelson (Publisher: Thieme 2008), counsel against using ultrasound frequencies below 7 to 7.5 MHz when seeking to achieve acceptable breast images. These books explain that much higher ultrasound frequencies, if possible as high as 12 MHz, should be used for breast imaging. “Guidelines from IBUS (International Breast Ultrasound School) for Ultrasonic Examination of the Breast” (Edited by Helmut Madjar et al; Published in European Journal of Ultrasound 1999; Vol. 9, pages 99-102) also recommends not using ultrasound frequencies below 7.5 MHz for breast imaging. However, breast ultrasound imaging at higher frequencies presents challenges because the ultrasound attenuation of breast tissue increases rapidly with ultrasound frequency, as shown by D'Astous and Foster (published in Ultrasound in Med. & Biol. 1986; Vol. 12, pages 795-808) (hereinafter “D'Astous and Foster”). With an attenuation coefficient of 1 to 2 dB/cm-MHz respectively for breast cancer and parenchyma tissues, at an ultrasound frequency of 7 MHz the resulting attenuation would reach the undesirable range of 42 to 84 dB for a 6 cm thickness of breast tissue. FIGS. 2-37 on page 34 of Stavros 2004 shows a penetration depth of around 3.5 cm for a breast ultrasound image obtained at 12 MHz. Current commercially available FFBUs are believed to operate in the range of ultrasound frequencies from 8 MHz to 14 MHz in order to obtain acceptable image quality for the range of breast sizes.
The known current commercially available FFBU scanning devices are rectilinear scanners, with scanning lines essentially parallel to each other as explained above. A significant challenge in these scanners is trying to fit a rectangular scan area over a round breast. Frequently each breast has to be scanned two to five or more times in overlapping set of scans. Even with good image stitching techniques, such as “Rapid image stitching and computer-aided detection for multipass automated breast ultrasound” reported by RF Change et al. (published in Medical Physics 2010; Vol. 37, pages 2063-2073), it is difficult to accurately stitch several separate scans of a breast into one single set forming a single image. Thus, a current practice of reading images of commercial FFBU is to view each of the several scans separately and independently as each scan covers different, although partially overlapping, parts of the breast. As a result, such multiple scans for each breast would require in longer interpretation times by physicians. Another problem for such multiple scans is an increase in the time for each patient in the scan examination room, which has a direct negative impact on: (1) patient throughput; and (2) revenue generation per FFBU per year.
There is a proposal for non-rectilinear FFBU scanning in WO 03/103500, which is not believed to have been commercially implemented. The reference proposes the use of a cone-shaped tissue molding element having a hole through which an ultrasonic transducer scans the breast as the molding element rotates relative to the breast. The figure in the reference appears to show that the wall of the molding element converges at an angle of about 90°. In comparison to one or more of the preferred embodiments described herein, where the scanned breast is flattened against the patient's chest wall, using such a 90° molding element would mean scanning through a much greater thickness of breast tissue. This would bring about two major shortcomings: (1) poorer image quality; and (2) limited range in size of breasts that can be scanned. This is because lower ultrasound frequency would have to be used for the greater thickness of scanned breast tissue, particularly in the case of larger breasts that would require ultrasound frequency below the minimum recommended 7.5 MHz. Early FFBU developments involving laterally compressed breasts (as in mammography), such as discussed in Pat. Publ. US 2006/0173303 A1, produced images of lower quality than current devices that scan a chestwardly compressed breast because lower ultrasound frequencies had to be used for larger breasts in such early development FFBU devices, and resulted in a change-over to chestward compression. Additional issues arise in the rectilinear scanning devices referred to above and in the devices discussed in WO 03/103500, as would be readily apparent to one skilled in the art in view of the disclosure in this patent specification.